1. Field of the Invention
The present invention relates to a welding method for a semiconductor material such as single crystal or polycrystal silicon (hereinafter referred to as xe2x80x9cSixe2x80x9d), gallium (hereinafter referred to as xe2x80x9cGaxe2x80x9d) or arsenic (hereinafter referred to as xe2x80x9cAsxe2x80x9d), wherein the semiconductor materials are welded by means such as electron beam, laser beam, or arc.
2. Description of the Related Art,
The physical and electrical properties of Si, when used as a semiconductor, have made it very popular. Si is especially utilized as a bulk material for substrates of DRAMs (Direct Random Access Memories) or MPUs (Micro Processor Units). The demand therefor is growing mostly for use as wafers. The diameters of wafers are becoming larger in order to produce more circuits in a single operation. Si ingots, of which wafers are made, currently exceed 200 kg in weight. As a consequence of this weight, it is necessary to consider how to hold and transport such ingots during subsequent manufacturing processes.
Besides the use of Si in semiconductors, it is often used also for manufacturing jigs for handling products. This use arises from the need to avoid contamination of metal in the products. The difficulty of manufacturing such jigs increases with increases in the weight and diameter of the ingots.
Though one may consider processing the Si ingots themselves to enable transport thereof, such a measure is attended by further problems such as higher costs and complication of manufacturing processes that need to be solved, since Si is a material that is hard to process and also requires certain treatments such as post-process washing.
For obtaining high-performance silicon wafers, thermal treatment at high temperature or film forming is performed by batch processing. Metallurgical tools for holding wafers during such batch processing have conventionally been made of quartz. However, single-crystal silicon boards are increasingly used to satisfy the need for wafers of larger diameter. The use of such boards is advantageous because the characteristics of the boards are identical to those of the wafers. As a result, use of single-crystal silicon boards are effective in decreasing the contamination level of the wafers.
However, it is presented a drawback in that these boards need to be manufactured through cutting from silicon ingots of single crystal whereby mechanical processing of these becomes quite expensive. Thus, it is desirable to find a way to freely connect these components in order to improve the degree of freedom for processing them.
Japanese Patent Application No. 3-107853 (1991) discloses a connection technique for wafers in which SOI wafers are manufactured by pasting wafers. Such a connection technique is employed with the aim of improving electric characteristics. This technique differs from the purpose of the present invention which is directed to structural characteristics.
Semiconductor materials such as Si are very brittle and are destroyed when subjected to high thermal gradients applied thereto. Accordingly, it was believed impossible to join semiconductor materials such as Si by welding.
It is an object of the invention to provide a welding method for semiconductor materials which overcomes the drawbacks of the prior art.
The inventors have achieved the present invention based on the fact that melt welding of semiconductor materials such as Si is possible while limiting metallic contamination or other contamination, such as oxidation, by employing a heat source of high energy density. The present invention enables welding of brittle semiconductor materials such as Si which were considered to be impossible to weld due to their very brittle characteristics. Utilizing a heat source of high energy density makes it possible to weld members made of semiconductor materials such as Si which could only be manufactured using mechanical processing techniques in the prior art. Besides joining members of Si, the present invention permits the manufacture of members having shapes which would be impossible to product using mechanical processing. Thus, the present invention improves the degree of freedom for processing semiconductor materials.
According to the welding method for semiconductor materials of the present invention, a heat source of high energy density is employed for performing melt welding of semiconductor materials. The semiconductor materials may include Si, Ga or As. The heat source of high energy density may includes electron beam, laser beam, plasma-arc or arc welding techniques.
Adequate control during partial melting of members to be welded is difficult using ordinary type heat sources of low energy density. A substantial amount of heat must be applied to achieve suitable melting. This produces such a large heat shock in semiconductor materials such as Si, before sufficient partial melting occurs, that at least partial destruction of the members to be joined results. Utilizing a heat source of high energy density as in the present invention, partial heating and melting at very small energy occurs over a limited area. The portion subjected to melting can be gradually expanded while restricting the total amount of heat input. This reduces the heat shock imparted to the semiconductor materials such as Si to realize a desired melting portion. It should be noted that, while it is preferable that the heat source have a high energy density of not less than 10 kw/cm2, the present invention is not limited to this value.
In the welding method for semiconductor materials according to the invention the application of energy progresses outward from the initial point at the time of starting welding. The energy output is decreased toward the end of the welding operation.
Since the energy output can be progressed or decreased to achieve desired output values, cracks due to thermal stress resulting from abrupt temperature distribution changes are reduced. Since output control is easy especially with electron beams or laser beams, partial heating is enabled while freely controlling the temperature increase/decrease rate.
The welding method for semiconductor materials according to the invention includes preheating the workpiece before welding. The preheating temperature is preferably not less than 300xc2x0 C., and more preferable, not less than 600xc2x0 C.
Preheating increases the area in which satisfactory conditions for welding exists. While Si is very brittle at room temperature, it has a ductility similar to steel when it is heated to not less than 600xc2x0 C. When preheating is performed, the cooling speed of the welding portion decreases as a matter of course. Thus, thermal stress owing from partial and abrupt shrinkage is reduced. Further, the region in which the temperature exceeds 600xc2x0 C. is expanded beyond the proximity of the welding portion, whereby not only relief of thermal stress is achieved but also ductility is improved in portions at which the thermal stress is large, so that probability of cracking is decreased.
The welding method for semiconductor materials according to the invention includes performing the welding while adding a filler material which is identical to the semiconductor material.
Referring now to FIG. 1, an effective means for relieving thermal shocks is a method in which a filler material 1, which is a material identical to the base material such as Si in a form of a stick, wire or powder, is added to a welding portion 2. Utilizing Si materials or the like as filler elements 1, most of the energy of the heat source is consumed for melting the filler element 1 whereby the amount of heat input to the members 3, 3 being welded, is decreased. Thus, thermal shock applied to the members 3, 3 is decreased and favorable welding without destruction can be achieved.
For electron beam welding, electricity must pass through the Si member. However, resistivity values of Si vary greatly depending on the amount of impurities included therein, and may reach up to 10,000 ohm-cm at room temperature. Therefore, it may be that welding is badly affected by electrification in that beams are deflected from the welding portion. Such problems can be coped with by grounding guide tip portion A of the filler element 1 as well as the members 3, 3 as shown in FIG. 1. When the thermal capacity of the filler element 1 is small, its temperature is easily raised to present favorable characteristics as an electric conductor, whereby the above described problems are reliably eliminated.
The welding method for semiconductor materials according to the invention includes periodical deflection control of a heating concurrently with application of heat from heat source of high energy density.
For preventing contamination of the welding portion by metal or oxidation, void-free welding using electron beams or laser beams is achieved by degassing of a molten pool utilizing the periodical deflection of the heat source (zone). Moreover, by welding under decreased pressure, favorable welding of good quality is performed by further aiding prevention of oxidation and contamination.
As described so far, the present invention performs welding of semiconductor materials such as Si which was considered to be impossible to weld due to their very brittle characteristics. The welding is achieved by utilizing a heat source of high energy density, to permit welding of members made of semiconductor materials such as Si which could heretofore be manufactured only through mechanical processing. In addition, the present invention permits manufacturing of members having shapes which could not be obtained through mechanical processing. Consequently, the degree of freedom for processing semiconductor materials is remarkably improved.
In highly brittle materials such as Si, notches, such as undercuts in welding beads, decrease the strength of the material. It is thus required to reduce occurrence of undercuts. The techniques of the present invention achieve such reduction in undercuts. That is, by performing preheating, the wettability of the molten pool and of heated portions that are not melted is improved, thereby reducing or preventing undercuts. The addition of filler elements is also effective in preventing undercuts. Further, performing periodic deflection control of the heat zone also reduces the occurrence of undercuts by vibration of the molten pool etc.
When using semiconductor materials such as Si, protrusions may be formed at end portions of the welding which are due to directional solidification. Such protrusions may decrease the strength of the welded structure. However, by decrement the energy output near the end of welding, the size of the molten pool is gradually decreased, whereby the formation of protrusions is reduced or eliminated. As a result, the reduction in strength can be prevented.
Briefly stated, the present invention provides a method for welding two overlapped semiconductor materials by melt welding under the influence of a heat source of high energy density. The energy output of the heat source is ramped up slowly at the beginning of welding and ramped down slowly at completion of welding. In one embodiment the semiconductor materials are preheated before welding. In another embodiment, the effective position of the heat source on the semiconductor materials is periodically deflected during welding.
According to an embodiment of the invention, there is provided a method for welding semiconductor materials comprising: positioning first and second semiconductor materials to be welded in proximity with each other, and applying heat from a heat source of high energy density to perform melt welding of the semiconductor materials.
According to a feature of the invention, there is provided a method for welding first and second members of a semiconductor material comprising: positioning said first and second members adjacent each other with surfaces to be welded facing each other, preheating the first and second members to a preheat temperature of at least 300xc2x0 C., applying a heat source of high energy density to the surfaces, the step of applying including increasing an energy output of the heat source over a first substantial period at a beginning of said welding, the step of applying further including decreasing said energy output over a second substantial period at an end of the welding, and periodically moving a contact point of the heat source on the first and second members during the welding.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.